The answer is China.
However, while it is true Chinese men made up a large number of miners, overall, there was a large mix of migrants from within the US and other parts of the world as well.
European immigrants who had just reached the United States from the east coast, quickly set out for the Oregon Gold Rush as well.
This was part of the larger Gold Rush taking place in the States of Montana, Idaho and Colorado in the 19th Century.
The Gold Rush was one of the reasons for these far flung areas becoming permanently settled.